1. Technical Field
The present invention relates to a hearing aid that combines noise suppression processing with nonlinear compression processing.
2. Background Art
A conventional hearing aid comprises an A/D converter for converting analog input signals produced according to input sound into digital input signals, a frequency characteristic processing means for adjusting the frequency characteristics of digital input signals, an amplifier for amplifying digital input signals, a D/A converter for converting digital input signals into analog sound signals and outputting the analog sound signals, a control signal input/output means for inputting and outputting control signals, and so forth.
With a conventional hearing aid, however, inputted sound is amplified without making any distinction between speech and sounds other than speech, and the amplified sound is outputted to the person wearing the hearing aid. Accordingly, when environmental noise other than speech becomes loud, this may become uncomfortable for the person wearing the hearing aid. In view of this, technology has been proposed for controlling the outputted sound by taking ambient sound into account.
For example, a technique has been proposed in which noise is suppressed by spectrum subtraction (SS), and the amplification ratio is varied according to the ratio between the signal power in a non-speech segment and the signal power of the inputted sound (see, for example, Patent Literature 1). Spectrum subtraction is a noise suppression processing method in which just the noise component is subtracted from a digital input signal by statistical estimation of the noise level of a non-speech segment.
Another technique has been proposed in which the compression and amplification characteristics are varied by detecting the degree of steadiness of environmental noise (see, for example, Patent Literature 2). The degree of steadiness referred to here is an index that expresses short-term fluctuations in power. In general, steady noise with little power fluctuation, such as at an air-conditioning equipment, has a high degree of steadiness, while noise that fluctuates sharply in power, such as in a sheet-metal plant, has a low degree of steadiness.
Another technique has been proposed in which the system switches between directional control and spectrum subtraction according to the environmental noise (see, for example, Patent Literature 3). Directional control is executed using a directional microphone or a plurality of non-directional microphones. When a directional microphone is used, the SN ratio (signal to noise ratio) can be improved by lowering the sensitivity of the microphone in everything but the forward direction, while leaving the sensitivity unchanged in the forward direction. When a plurality of non-directional microphones are used, sound from ahead can be emphasized by correcting any offset in the time at which speech was inputted to the plurality of microphones, and adding together the plurality of input signals.
Yet another technique has been proposed in which, in directional control, the system smoothly switches the sound reception characteristics of the hearing aid between omnidirectional characteristics and directional characteristics (see, for example, Patent Literature 4). Switching the sound reception characteristics is accomplished by performing controlled attenuation of a signal derived from the input signals (Xfront and Xback) from first and second microphones, and controlled retardation of time or phase, and then producing an overall synthetic signal (Y) by using an adjustable attenuation control parameter (omni) and retardation (T).